1. Field of the Invention
The present invention relates to an optical microscope. In particular, it relates to a shear force feedback system, a large area scanning-tip near field optical microscope using said system, and a scanning method for said microscope.
2. Description of Prior Art
The art of the near field optical microscopy transcends the restrictions of optical diffraction. It allows the observation of optical properties as small as several hundred angstroms on the surface of the sample, permitting its use in the fields of the sub-micron technology and the biology. However, while using this method, it is necessary to maintain a distance between the probe and the surface of the sample of less than several hundred angstroms. Thus, near-field optical microscopy requires a feedback control to keep this distance. At present, the feedback control mechanism is either optical or non-optical. In prior arts, there are many mechanisms using optical techniques such as "Near-Field Optics: Microscopy, Spectroscopy, and Surface Modification Beyond the Diffraction Limit, " written by E. Betzig and J. K. Trautman, Science 257, 189(1992), and "Combined shear force and near-field scanning optical microscopy," disclosed by E. Betzig, P. L. Finn and J. S. Weiner, Appl. Phys. Lett. 60, 2484 (1992), and "Minimum detectable displacement in near-field scanning optical microscopy," of F. F Froehlich and T. D. Milster, Appl. Phys. Lett. 65, 2254(1994). There are also many mechanisms using non-optical technique such as "Piezoelectric tip-sample distance control for near field optical microscopes," disclosed by K. Karrai and R. D. Grober, Appl. Phys. Lett. 66, 1842(1995), and "A non-optical tip-sample distance control method for near-field scanning optical microscopy using impedance changes in an electromechanical system," disclosed by J. W. P. Hsu, M. Lee and B. S. Deaver, Rev. Sci. Instrum. 66, 3177 (1995). Optical methods have the benefit of simple and stable utility and are widely used by researchers.
FIG. 1 shows a prior art of a near-field optical microscope using an optical mechanism, which is described below:
At first, a piezoelectric material 10 actuates a fiber probe 12 to vibrate in a direction parallel to the surface (i.e., x direction).
Then the light beam is focused on the tapered region of the tip end of the fiber probe by the lens 14, a part of the light beam reflected by the tip end being focused by the lens 16.
The light focused by the lens 16 is then detected by the photo-detector 18. The intensity of the light brings the information regarding to the vibration amplitude of the tip position.
The phase-locked amplifier is used to measure the small signal of the amplitudes or the phases of the vibration. Finally, the varieties of the amplitudes or the phases serve as the feedback signal of the distance between the fiber probe and the surface of the sample.
The optical method given above has been disclosed in "Direct measurements of the true vibrational amplitudes in shear force microscopy," C. C. Wei, P. K. Wei and W. S. Fann, Appl. Phys. Lett. 67, 3835 (1995).
FIG. 2 shows a part of the light beam being reflected and passing through the lens 24 to the photo-detector 26 while a light beam is focused on the tip of the probe 22 in the transversal direction (x direction). The light beam (P.sub.dc (x)) reflected to the photo-detector 26 changes in intensity as the probe 22 moves. The relationship between the variation (P.sub.ac (x)) and the small displacement(.DELTA.x) is given below: EQU P.sub.ac (x)=.vertline.P.sub.dc (x+.DELTA.x)-P.sub.dc (x).vertline..apprxeq..vertline.P'.sub.dc (x).vertline..DELTA.X (1)
The distribution of a conventional laser light focus beam is a Gaussian distribution, i.e., exp ##EQU1##
so ##EQU2##
The measurement of the amplitude (P.sub.ac (x)) has a maximum value at the position ##EQU3##
so ##EQU4##
This value is inversely proportional to the size of the focus spot and the position of the maximum value is at ##EQU5##
so a preferred amplitude signal can be obtained if the size of the focus spot is small.
The optical feedback control method is necessary to put the fiber probe near the focus spot of the light beam, and the size of the focus spot should be as small as possible to obtain a higher value signal. This system provides only a small moving range for the probe on the parallel plane (x-y plane) when the probe scans the surface of the sample. Thus, in a conventional near-field optical microscope, the probe is not moved while scanning the sample's surface. Instead, a piezoelectric crystal is used to move the scanning sample along the parallel plane of the surface. This system is so called the scanning sample system.
However, when the sample needs to be fixed or is too heavy or big, it is impossible to use the sample in this system since the piezoelectric crystal can not move it. In this situation, it is necessary to use a scanning tip system in which the system moves along the x-y plane while controlling the distance between the fiber probe and the surface of the sample.